Procedure:

Reconstitute the target protein (0.1-1 mg) in 10 ul of 6 M Guanidine- HCl, 25 mM Ammonium Bicarbonate, pH 8.0 (The amount of protein depends on how complex your protein mixture is, you need to obtain a final concentration of >10 fmol/ l of each protein in the protein mixture after you finish the final step of the digestion).